The present invention relates to an apparatus for measuring temperature by light with use of a fluorescent substance.
The terms "fluorescence" and "fluorescent substance" are used in this specification and the appended claims as including "phosphorescence" and "phosphorescent substances". Further, as to the luminescence of the fluorescent substance, luminescence during exciting is termed fluorescence, as distinguished from afterglow which means luminescence after the cessation of excitation.
Methods of measuring temperatures with use of light, wherein no electric signals are used, have the feature of being totally free of the infuence of noise due to electromagnetic fields, and various apparatus incorporating optical fibers have been developed for such methods. Above all, temperature measurement by making use of the luminescence of a fluorescent substance has the advantage that because the wavelength of the light for exciting the fluorescent substance differs from that of the light emitted by the substance, the output light from the measuring system can be separated from the input light thereto, that is, the fluorescence and afterglow of the fluorescent substance can be separated according to wavelength. It is therefore possible to transmit input and output light with a single optical fiber and to use a compact light transmission system.
The techniques for measuring temperatures using fluorescent substances are divided generally into those making use of the temperature dependence of the intensity of fluorescence during exciting, and those resorting to the temperature dependence of the duration of afterglow after the cessation of exciting. Any of the conventional temperature measuring techniques utilizes fluorescence or afterglow unused for and irrelevant to the measurement of temperature with respect to a time series. In addition to this common drawback, the conventional techniques for measuring temperature with use of fluorescence or afterflow have the following problems.
In measuring temperature by utilizing the variation in the intensity of fluorescence with temperature, exciting light of high intensity and fluorescence of low intensity are present at the same time, so that the two kinds of light must be spectrally or otherwise separated according to the wavelength. This requires an optical system and involves the drawback that it is difficult to provide measurements with a high S/N ratio.
On the other hand, in measuring temperature by making use of the variation in the intensity of afterglow with temperature, afterglow, which is the light emitted after the cessation of exciting, is detected. Accordingly this measuring method has the advantage that the exciting light exerts no influence on the measurement of afterglow in principle, permitting easy optical measurement. To determine the duration of afterflow, it is general practice to measure the period of time from the cessation of exciting or from the time when the intensity of afterglow decreases to 90% of the intensity at the time of cessation of exciting (i.e. peak intensity) to the time when the intensity decreases to 10% of the peak intensity. Since which points on the afterglow curve correspond to these measuring time points becomes apparent after the completion of the measuring procedure, all the signal components forming the afterglow curve are to be covered by a measuring process. Consequently the method has the drawback that the accuracy of measurement is subject to the influence of an unexpected or partial signal variation which could occur during the measuring process due to noise or the like. The method has another problem. To determine the duration of afterglow, the intensity of afterglow must be measured during the decay of afterglow, whereas it is difficult to measure low intensities of afterglow and therefore to determine the point in time when the afterglow intensity becomes zero without error. Thus it is impossible to accurately determine the duration of afterglow. Stated more specifically, the intensity of afterglow decreases as an exponential or hyperbolic function, and the variation of intensity of afterglow is very small where the intensity is very low. Accordingly the determination of the point in time when the afterglow intensity decreases to 10% of the peak value invariably involves a large error factor.